The use of inorganic corrosion inhibitors, e.g. metal salts and oxides either alone or in combination with various organic inhibitors such as the azoles and the phosphonic acids have been used in aqueous systems. It has been found in accordance with this invention, however, that certain amino alkylene phosphonic acids and the derivatives thereof and particularly the amino methylene phosphonic acids having increased number of methylene groups in combination with a water-soluble carboxylic acid having at least one hydroxyl group per molecule and its alkali metal salts have improved metallic corrosion inhibition. In addition, a combination of phosphonic acid and carboxylic acid or the salts thereof have been found to prevent the deposition of mineral scale normally encountered in aqueous systems.
Corrosion is generally defined as a destructive attack on the metal, e.g. iron, involving an electrochemical or chemical reaction of the metal with its environment. Specifically, an electrochemical attack on the metal surface is the wearing away or an under cutting of metal which is accelerated after the protective coating, e.g. the oxide films are removed by the corrosive medium. Corrosion includes cavitation and erosion where, in addition to an electrochemical reaction, the condition of the aqueous systems are such that the continuous flow causes cavities where high pressure areas develop causing pressure and shock resulting in pitted metal surfaces. This particular type of corrosion is found in water pumps, propellers, turbine plates, etc. Erosion of the metal surface, however, occurs when the medium contains suspended solids which impinge the surface of the metal as the fluid is transported, thereby removing the protecting film and exposing the metal to corrosion.
Various corrosion inhibiting compositions are now being used at threshhold levels in an attempt to control the deposition of mineral scale. At these levels, however, there is a tendency to increase the rate of corrosion and, therefore, stronger inhibitors at higher concentrations are necessary in order to obtain satisfactory inhibition. The use of these inhibitors, e.g. the chromates at higher concentrations is unsatisfactory particularly because of environmental restriction.
Compounds such as the chromates, zinc compounds, polyphosphates, silicates or the like are used to control scale and/or corrosion in cooling water systems. Recently, however, more Government restrictions have been placed on the use of inhibitors containing heavy metals. Moreover, some of the polyphosphates are objectionable because of degradation they combine with calcium resulting in the precipitation of calcium salts.
Thus, with the increase in regulation on the kind of compounds that can be used in water systems, there is a need for environmentally accepted inhibitors. The inhibitors of this invention comprising the amino alkylene phosphonic acids in combination with the water-soluble carboxylic acids substantially limit the rate of corrosion, extend the life of the equipment and minimize down time for maintenance purposes. Moreover, it was discovered that molybdenum compounds, when used in combination with the carboxylic acids and phosphonic acids, are satisfactory replacements for the chromates in that molybdates are not on the list of toxic compounds requiring Federal regulation. Thus, to avoid these and related problems, it has been discovered that certain alkylene phosphonic acids and the derivatives thereof in combination with effective amounts of water-soluble carboxylic acids having at least one hydroxy group per molecule and the alkali metal salts of said acids may be used alone or in combination with other known compounds in aqueous systems to prevent corrosion and scale formation.